FIG. 1 shows as an example a hybrid drive having an internal combustion engine ICE, a separating clutch K0, an electric motor EM, another separating clutch K2 and a transmission module TR. Separating clutch K0 is provided to separate internal combustion engine ICE from the drive train or to connect it thereto again. Thus, for example, in a purely electric motor drive using electric motor EM as the drive source, separating clutch K0 is disengaged and internal combustion engine ICE is shut down. In a transition from an electric motor drive to a hybrid drive in which both electric motor EM and internal combustion engine ICE are used as drive sources, internal combustion engine ICE may first be driven mechanically by separating clutch K0 to achieve a predefined rotational speed, for example. Separating clutch K0 is operated in a slip state in which it is not completely engaged. At the same time, separating clutch K0 is connected to the drive train on the side of the electric motor. It is therefore of crucial importance for driving comfort in particular to accurately control the slip of separating clutch K0 in hybrid mode. German Patent Application No. DE 105 40 921 A1 describes in this context a system for controlling a servo clutch, in which the clutch control is optimized.
The developing torque during engagement of separating clutch K0 shown in FIG. 1 may be detected by the method depicted in FIG. 2, for example. FIG. 2A shows a curve of a torque M of an electric motor over time, FIG. 2B shows a curve of a rotational speed of the electric motor over time, and FIG. 2C shows a curve P of the state of separating clutch K0, which may assume any states between a disengaged state and a completely engaged state. Curve P of the state of separating clutch K0 is determined by a curve of the engaged positions of separating clutch K0. Separating clutch K0 is thus in a slip state, for example, when it is only partially engaged and is in a disengaged state when it is completely disengaged.
As shown in FIG. 2A, the torque of the electric motor is increased linearly up to a resulting torque 201 and subsequently reduced again along a ramp. With the increase in torque, the rotational speed of the electric motor illustrated in FIG. 2B drops due to the increase in the clutch transfer torque, but it increases again with a decline in torque. As shown in FIG. 2C, separating clutch K0 is engaged slowly starting from a disengaged state 203 and is therefore in a continuous slip. Separating clutch K0 is engaged until reaching a position 205, in which the resulting developing torque 201 is established.
To detect the developing torque, internal combustion engine ICE is initially shut down and separating clutch K0 is disengaged, the electric motor torque on the main drive axle being kept as constant as possible. The rotational speed of the electric motor is kept constant at 500 rpm, for example, by a speed regulator. At point in time 207, separating clutch K0 is engaged slowly. The speed regulator then attempts to keep the speed of the electric motor constant by generating an additional torque, for example. For detecting the developing torque, at point in time 205 a position of separating clutch K0 is detected at which the electric motor torque has increased by 10 Nm, for example. In this way, the so-called contact point of separating clutch K0, at which the transmitted torque is 0 Nm, may also be detected.
One disadvantage of the method described above is that it may take approximately 3 s to 10 s until the developing torque has been detected during engagement of separating clutch K0. This is due to the fact that the speed of the engaging separating clutch K0 must be lower than the response rate of the speed regulator controlling the electric motor. Consequently, the engaged position of separating clutch K0 at point in time 205 may be detected only as a function of the torque, which has been increased by the speed regulator. In addition, another disadvantage is that separating clutch K0, which is in the slip state for a longer period of time, must withstand higher torques, and it may thus be damaged. For this reason, the learning range to be considered in order to detect the developing torque should have lower torques than these higher torques, so that the torque transmitted via separating clutch K0 is usually less than 10 Nm. It is thus impossible to detect higher rotational speeds and higher torque ranges at 50 Nm, for example, for detecting the developing torque of separating clutch K0. In addition, in the range in which the torque transmitted by separating clutch K0 is low, a higher mechanical tolerance and thus a lower engagement precision of separating clutch K0 are to be expected. For this reason, this method is performed in a mechanically unstable range, so that the developing torque during engagement of separating clutch K0 cannot be detected accurately.